MIMO, which is an abbreviation for Multiple-Input Multiple-Output, refers to a scheme capable of improving data transmission/reception efficiency using multiple transmit antennas and multiple receive antennas, instead of using a single transmit antenna and a single receive antenna. In other words, MIMO technology allows a transmitter or a receiver of a wireless communication system to use multiple antennas, so that capacity or performance can be improved. Herein, MIMO will be called multi-antenna.
In summary, multi-antenna technology applies technology of completing a whole message by gathering data fragments received via several antennas without depending on a single antenna path in order to for one whole message. Since MIMO technology can improve a data transmission rate in a specific range or increase system range at a specific data transmission rate, it is next-generation mobile communication technology which can be widely used in mobile communication terminals, relays, etc. Attention is being paid to this technology as a next-generation technology capable of overcoming limitations in mobile communication transmission capacity that has become a critical situation due to expansion of data communication.
Generally, when a transport channel experiences deep fading, unless a different version or a replica of a transmitted signal is additionally transmitted, a receiver can hardly determine the transmitted signal. A source corresponding to such different version or replica is called diversity which is one of the most significant factors contributing to reliable transmission over wireless channels.
The use of diversity can maximize data transmission capacity or data transfer reliability. A system implementing diversity using multiple transmit antennas and multiple receive antennas is called a MIMO system or a multi-antenna system.
To overcome performance degradation caused by channel fading of wireless communication, studies have been extensively conducted on a spatial diversity scheme using a MIMO system.
The MIMO system provides advantages of high data transmission rate, low error rate, and channel capacity increase by implementing two or more antennas in a transmitter and a receiver.
However, in spite of the advantages of the MIMO system, it is generally impossible to achieve the MIMO system in uplink due to restrictions such as size, weight, hardware complexity, etc.
As an alternative for this problem, a cooperative diversity scheme has been proposed. The object of the cooperative diversity scheme is for each terminal to obtain advantages of spatial diversity gain increase, error rate reduction, and channel capacity increase. To this end, the cooperative diversity scheme forms a virtual MIMO system using a relay or a femto cell so that neighboring terminals share resources such as an antenna and a frequency band of the relay. Thus, the advantages of the MIMO system can be obtained even through terminals having at least one antenna.
In this way, the cooperative diversity scheme is based on use of an additional relay.
However, a scheme capable of performing cooperative transmission using terminals without an additional relay is needed.